Process for production of honeycomb structure

ABSTRACT

A process for producing a honeycomb structure, which comprises adding water to a mixed raw material of a raw material powder and a binder, kneading the mixture to obtain a plastic mixture, molding the plastic mixture into a honeycomb shape to obtain a green honeycomb structure, drying the structure by a step including hot-air drying, and firing the resulting dried honeycomb structure, wherein the binder contains hydroxypropyl methyl cellulose as a major component. The process can produce a crack-free high-quality honeycomb structure rapidly at a low cost.

CROSS-REFERENCE TO RELATED APPLICATION

This is a 371 of PCT/JP01/04393, filed on May 25, 2001.

TECHNICAL FIELD

The present invention relates to a process for producing a honeycombstructure. More particularly, the present invention relates to a processfor producing a honeycomb structure, which can produce a crack-freehigh-quality honeycomb structure rapidly at a low cost.

BACKGROUND ART

Honeycomb structures have a structure in which a plurality ofthrough-holes are formed by thin partition walls, in a honeycomb shape.Of such honeycomb structures, those made of a ceramic material [e.g.silicon carbide (SiC)] are in use as an exhaust gas purificationapparatus for capturing and removing the fine particle carbon present inan exhaust gas emitted from a diesel engine, etc.

Honeycomb structures have hitherto been produced by a process whichcomprises mixing a ceramic material with a binder composed mainly ofmethyl cellulose, adding, to the resulting mixed raw material, a liquidmedium, ordinarily water in a given amount, kneading the mixture toobtain a plastic mixture, molding the plastic mixture into a honeycombshape to obtain a green honeycomb structure, subjecting the greenhoneycomb structure to hot-air drying, and firing the dried honeycombstructure.

In this production process, however, the rate of removing the liquidmedium component in the green honeycomb structure is small and the ratesof drying at the surface and inside of the green honeycomb structurediffer largely from each other; therefore, there has been a problem inthat the green honeycomb structure comes to have a strain caused by adifference in contraction on drying between the surface and inside ofthe green honeycomb structure and cracks are generated in the driedhoneycomb structure.

In contrast, a process is proposed in which a green honeycomb structureis dried by a drying step consisting of a combination of hot-air dryingand microwave drying or dielectric drying, to remove, by the microwavedrying or the dielectric drying, the most part of a liquid mediumpresent in the green honeycomb structure. In the case of a honeycombstructure using a non-electroconductive ceramic material (e.g.cordierite), the whole honeycomb structure can be dried rapidly anduniformly; therefore, the process is advantageous in that the generationof cracks caused by drying can be prevented significantly.

In the production process, however, when an electroconductive ceramicmaterial [e.g. silicon carbide (SiC)] is used, the applied microwave orthe like is absorbed at the surface of the green honeycomb structure anddrying is insufficient at the inside where the microwave or the likedoes not reach. Therefore, when the ratio of removal of the liquidmedium in the green honeycomb structure by microwave drying or the likeis high, for example, higher than 65%, a strain appears owing to adifference in contraction on drying between the surface and inside ofthe green honeycomb structure; as a result, there have been cases thatcracks are generated in the dried honeycomb structure or significantheat generation takes place at the surface of the green honeycombstructure, resulting in complete combustion of the binder during thedrying step.

On the other hand, when the ratio of removal of the liquid medium in thegreen honeycomb structure by microwave drying or the like is low, forexample, not higher than 65%, the rate of removal of the liquid mediumfrom the green honeycomb structure is small and a difference in dryingrate appears between the surface and inside of the green honeycombstructure; as a result, a strain appears as well in the molded structureowing to a difference in contraction on drying between the surface andinside of the green honeycomb structure and it may form cracks in thedried honeycomb structure.

Meanwhile, there are proposed processes for producing a honeycombstructure, wherein a green honeycomb structure is dried by vacuum dryingor freeze-drying and thereby the surface and inside of the greenhoneycomb structure can be dried uniformly regardless of the kind of theceramic material used (see Japanese Patent Nos. 2612878, 3015402, etc.).

In these production processes, however, since drying is conducteduniformly, an extremely long drying time and accurate control of dryingconditions are necessary; therefore, there have been problems in thatquick production is difficult and a high production cost is required.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the above problems andaims at providing a process for producing a honeycomb structure, whichcan produce a honeycomb structure rapidly at a low cost withoutgenerating cracks. The present inventor made a study in order to achievethe above task and, as a result, found out that by drying a greenhoneycomb structure prepared from a mixed raw material using a bindercontaining, as a major component, a hydroxypropyl methyl cellulosehaving particular properties, especially a hydroxypropyl methylcellulose having a water separation ratio of 40% or more, rapid anduniform drying of green honeycomb structure is possible regardless ofthe kind of the raw material powder used. The present invention has beencompleted based on the finding.

According to the present invention there is provided a process forproducing a honeycomb structure, which comprises adding water to a mixedraw material of a raw material powder and a binder, kneading the mixtureto obtain a plastic mixture, molding the plastic mixture into ahoneycomb shape to obtain a green honeycomb structure, drying thestructure by a step including hot-air drying, and firing the resultingdried honeycomb structure, characterized in that the binder containshydroxypropyl methyl cellulose as a major component (hereinafter, thisprocess may be referred to simply as “the first embodiment).

In the production process according to the present invention, thehydroxypropyl methyl cellulose which is a major component of the binder,preferably has a water separation ratio specified by the followingformula (1), of 40% or more.water separation ration=(Wb−Wa)/(Wb)×100   (1)(In the formula, Wa is a gel weight after water separation and Wb is agel weight before water separation).

The binder may further contain at least one kind selected from the groupconsisting of methyl cellulose, hydroxyethyl cellulose, carboxymethylcellulose and a polyvinyl alcohol.

In the production process, the mixed raw material is preferably amixture of 68 to 98% by weight of a raw material powder, 2 to 15% byweight of a binder and 0 to 30% by weight of additives.

In the production process, the drying step preferably includes a step ofhot-air drying capable of removing the liquid medium in the greenhoneycomb structure by an amount of 35% or more and more preferablyfurther includes a step of microwave drying and/or dielectric drying.

Also in the production process according to the present invention, thereis provided a process for producing a honeycomb structure, characterizedin that the raw material powder contains SiC in an amount of 50% or more(hereinafter, may be referred to as “the second embodiment”).

In the second embodiment as well, the hydroxypropyl methyl cellulosewhich is a major component of the binder, preferably has a waterseparation ratio specified by the following formula (1), of 40% or more.water separation ration=(Wb−Wa)/(Wb)×100   (1)(In the formula, Wa is a gel weight after water separation and Wb is agel weight before water separation).

In this embodiment as well, the binder may contain, in addition tohydroxypropyl methyl cellulose, at least one kind selected from thegroup consisting of methyl cellulose, hydroxyethyl cellulose,carboxymethyl cellulose and a polyvinyl alcohol.

In this embodiment, the mixed raw material is preferably a mixture of 68to 98% by weight of a raw material powder containing SiC in an amount of50% or more, 2 to 15% by weight of a binder and 0 to 30% by weight ofadditives.

In this embodiment as well, drying of the green honeycomb structurepreferably includes a step of hot-air drying capable of removing theliquid medium in the green honeycomb structure by an amount of 35% ormore and more preferably further includes a step of microwave dryingand/or dielectric drying.

In the production process according to the present invention, there isalso provided a process for producing a honeycomb structure,characterized in that the raw material powder contains SiC and a metalin a total amount of 90% or more (hereinafter, may be referred to as“the third embodiment”).

In the third embodiment as well, the hydroxypropyl methyl cellulosewhich is a major component of the binder, preferably has a waterseparation ratio specified by the following formula (1), of 40% or more.Water separation ration=(Wb−Wa)/(Wb)×100   (1)(In the formula, Wa is a gel weight after water separation and Wb is agel weight before water separation).

Of course, in this embodiment as well, the binder may contain, inaddition to hydroxypropyl methyl cellulose, at least one kind selectedfrom the group consisting of methyl cellulose, hydroxyethyl cellulose,carboxymethyl cellulose and a polyvinyl alcohol.

In this embodiment, the mixed raw material is preferably a mixture of 68to 98% by weight of a raw material powder containing SiC and a metal ina total amount of 90% or more, 2 to 15% by weight of a binder and 0 to30% by weight of additives.

In this embodiment as well, drying of the green honeycomb structurepreferably includes a step of hot-air drying capable of removing theliquid medium in the green honeycomb structure by an amount of 35% ormore and more preferably further includes a step of microwave dryingand/or dielectric drying.

BEST MODE FOR CARRYING OUT THE INVENTION

The modes for carrying out the present invention are specificallydescribed below.

1. First Embodiment

In the first embodiment of the present invention, first, a raw materialpowder is mixed with a binder containing hydroxypropyl methyl celluloseas a major component, to prepare a mixed raw material.

The raw material powder used in the first embodiment contains a ceramicmaterial such as inorganic oxide, inorganic carbide, inorganic nitride,inorganic boride, semiconductor and the like, and its mixture with ametal or the like.

As the ceramic material contained in the raw material powder, there canbe mentioned, for example, those containing at least one kind selectedfrom the group consisting of silicon, titanium, zirconium, siliconcarbide, boron carbide, titanium carbide, zirconium carbide, siliconnitride, boron nitride aluminum nitride, aluminum oxide, zirconiumoxide, mullite, raw materials for cordierite, aluminum titanate, sialon,kaolin and talc.

The ceramic material may be composed of any of various crystallinesubstances (e.g. α-crystalline substances and β-crystalline substances)and amorphous substances and may be a mixture of two or more kinds ofcrystal systems.

As the metal contained in the raw material powder, there can bementioned those containing at least one kind selected from the groupconsisting of copper, aluminum, iron, nickel and silicon. These metalscan be used singly or in combination of two or more kinds.

The metal is contained in the raw material powder in an amount ofpreferably 5 to 80% by weight, more preferably 10 to 60% by weight,particularly preferably 20 to 40% by weight.

The binder used in the present invention contains hydroxypropyl methylcellulose as a major component. Incidentally, the major component refersto that it is contained in an amount sufficient to exhibit an intendedeffect in the production process according to the present invention.

By using hydroxypropyl methyl cellulose as a major component of thebinder, rapid and uniform drying is possible particularly in hot-airdrying; rapid and uniform drying is possible in a drying step describedlater, not only when cordierite of low electroconductivity is used inthe raw material powder but also when SiC of high electroconductivity orthe like is used in the raw material powder; and a crack-freehigh-quality honeycomb structure can be produced.

Therefore, the binder may be hydroxypropyl methyl cellulose alone or amixture of hydroxypropyl methyl cellulose with other composition.However, preferred is a binder containing hydroxypropyl methyl cellulosein an amount of 70% by weight or more, more preferred is a bindercontaining the cellulose in an amount of 80% by weight or more,particularly preferred is a binder containing the cellulose in an amountof 90% by weight or more.

When the amount is less than 70% by weight, rapid and uniform drying isdifficult in hot-air drying described later; as a result, the honeycombstructure obtained may generate cracks.

As the hydroxypropyl methyl cellulose which is a major component of thebinder, there can be mentioned one having a methoxy substitution degreeof 28.1 to 30.0% by weight and a hydroxypropyl substitution degree of5.7 to 12.0% by weight. Further, the hydroxypropyl methyl cellulosewhich is a major component of the binder, preferably has a waterseparation ratio of 40% or more.

When the water separation ratio is less than 40%, rapid drying byhot-air drying described later is difficult, a strain appears duringdrying owing to a difference in contraction on drying, between theinside and outside of green honeycomb structure, and the resultinghoneycomb structure may generate cracks.

Here, “water separation ratio” is an index of the easiness of drying ofcellulose derivative and was measured by a method shown below.

First, there was prepared an aqueous solution which contains 1% byweight of methyl cellulose and 1% by weight of a cellulose derivative tobe evaluated (2% by weight in total); 50 g of this aqueous cellulosederivative solution was heated in a closed vessel at 90° C. for 1 hour.In this case, the weight of the cellulose derivative gel formed byheating is measured and taken as a gel weight before water separation.

Then, the cellulose derivative gel was allowed to stand on a net placedin another closed vessel.

As the crystal growth aid, there can be mentioned, for example,magnesia, silica, yttria and iron oxide; as the dispersing agent, therecan be mentioned, for example, ethylene glycol, dextrin, fatty acid soapand a polyalcohol; as the pore former, there can be mentioned, forexample, graphite, wheat flour, starch, a phenolic resin and apolyethylene terephthalate.

The mixed raw material used in the present invention is preferably amixture of 68 to 98% by weight of a raw material powder, 2 to 15% byweight of a binder and 0 to 30% by weight of additives; more preferablya mixture of 70 to 95% by weight of a raw material powder, 3 to 10% byweight of a binder and 0 to 25% by weight of additives; particularlypreferably a mixture of 75 to 90% by weight of a raw material powder, 3to 8% by weight of a binder and 0 to 20% by weight of additives.

When the content of the raw material powder is less than 68%, sinteringmay be insufficient and, when the content is more than 98%, molding maybe impossible. When the content of the binder is less than 2%, moldingmay be impossible and, when the content is more than 15%, cracks may begenerated in firing. When the content of the additives is more than 30%,cracks may be generated in firing.

In the process for producing a honeycomb structure according to thepresent invention, next, the above-mentioned mixed raw material is mixedwith a given amount of water, followed by kneading, to obtain a plasticmixture.

The amount of water mixed is preferably 10 to 40 parts by weight, morepreferably 15 to 35 parts by weight, particularly preferably 15 to 30parts by weight relative to 100 parts by weight of the mixed rawmaterial.

When the amount is less than 10 parts by weight, the plasticity of theraw material composition is insufficient, making its molding difficult.When the amount is more than 40 parts by weight, the raw materialcomposition has fluidity, making its molding difficult as well.

In the process for producing a honeycomb structure according to thepresent invention, then, the above-mentioned plastic mixture is moldedinto a green honeycomb structure.

There is no particular restriction as to the method of molding. However,extrusion is preferred from the standpoint of mass production.

Specifically, there can be mentioned, for example, a molding methodwhich comprises molding a plastic mixture into a cylindrical shape usinga vacuum pug mill and then molding the cylindrical shape into ahoneycomb shape using a ram extruder.

In the process for producing a honeycomb structure according to thepresent invention, then, the green honeycomb structure is dried by astep including hot-air drying, preferably a step including hot-airdrying capable of removing a liquid medium present in the greenhoneycomb structure by 35% or more.

In the present invention, since a binder containing hydroxypropyl methylcellulose as a major component is used, rapid and uniform drying ispossible even when there is used a step including hot-air drying (inparticular, a step including hot-air drying capable of removing a liquidmedium present in the green honeycomb structure by 35% or more), and acrack-free honeycomb structure can be produced rapidly at a low cost.

The drying step may include, besides the above, for example, at leastone kind selected from the group consisting of microwave drying,dielectric drying, reduced pressure vacuum drying and freeze-drying.

The drying step preferably includes, of these drying, microwave dryingand/or dielectric drying, for rapid drying.

In the process for producing a honeycomb structure according to thepresent invention, lastly, the dried honeycomb structure obtained by theabove-mentioned drying step is fired.

There is no particular restriction as to the method of firing, and thefiring can be conducted by an ordinary method.

2. Second Embodiment

In the second embodiment of the present invention, first, a raw materialpowder containing 50% by weight or more of silicon carbide (SiC) ismixed with a binder containing hydroxypropyl methyl cellulose as a majorcomponent, to prepare a mixed raw material.

Thereby, a green honeycomb structure can be dried rapidly and uniformlyas described later, even when there is used, for production of ahoneycomb structure, a raw material powder containing 50% by weight ormore of silicon carbide (SiC) which has a very high mechanical strengthand a high electrical conductivity and is difficult to subject tomicrowave drying or dielectric drying.

The raw material powder used in the second embodiment can contain,besides silicon carbide (SiC), a ceramic material containing at leastone kind selected from the group consisting of silicon, titanium,zirconium, silicon carbide, boron carbide, titanium carbide, zirconiumcarbide, silicon nitride, boron nitride, aluminum nitride, aluminumoxide, zirconium oxide, mullite, raw materials for cordierite, aluminumtitanate, sialon, kaolin and talc.

The ceramic material may be any of various crystalline substances (e.g.α-crystalline substances and β-crystalline substances) and amorphoussubstances and may be a mixture of two or more kinds of crystal systems.

The binder used in this embodiment is the same as described in the firstembodiment, but preferably contains, as a major component, ahydroxypropyl methyl cellulose having a water separation ratio of 40% ormore because a raw material powder of high electrical conductivity isused.

In this embodiment, then, the above-mentioned mixed raw material ismixed with water, followed by kneading, to obtain a plastic mixture; theplastic mixture is molded into a honeycomb shape to obtain a greenhoneycomb structure.

The preparation and molding of the plastic mixture are conducted in thesame manner as in the first production process.

In the second embodiment of the present invention, then, the greenhoneycomb structure is dried. The drying of the green honeycombstructure can be conducted, for example, by a step containing at leastone kind selected from the group consisting of hot-air drying, microwavedrying, dielectric drying, reduced pressure drying, vacuum drying andfreeze-drying.

In particular, there is preferred a step containing hot-air dryingcapable of removing a liquid medium in the green honeycomb structure by35% or more and, for quicker drying, there is more preferred a stepfurther containing microwave drying and/or dielectric drying.

In this embodiment of the present invention, lastly, the dried honeycombstructure obtained by the above drying step is fired.

The method of firing is the same as in the first embodiment.

3. Third Embodiment

In the third embodiment of the present invention, first, a raw materialpowder containing SiC and a metal in a total amount of 90% or more ismixed with a binder containing hydroxypropyl methyl cellulose as a majorcomponent, to prepare a mixed raw material.

Thereby, various dryings such as hot-air drying, microwave drying,dielectric drying and the like, described later can be conducted rapidlyand uniformly even when there is used, in order to produce a honeycombstructure, a raw material powder containing, in a total amount of 90% ormore, a metal and SiC having a very high electrical conductivity anddifficult to subject to microwave drying and dielectric drying.

As the metal contained in the raw material powder, there can bementioned one containing at least one kind selected from the groupconsisting of copper, aluminum, iron, nickel and silicon. These metalscan be used singly or in combination of two or more kinds.

The metal is contained in the raw material powder in an amount ofpreferably 5 to 80% by weight, more preferably 10 to 60% by weight,particularly preferably 20 to 40% by weight.

The raw material powder used in the third embodiment can contain,besides SiC and a metal, a ceramic material containing at least one kindselected from the group consisting of silicon, titanium, zirconium,silicon carbide, boron carbide, titanium carbide, zirconium carbide,silicon nitride, boron nitride aluminum nitride, aluminum oxide,zirconium oxide, mullite, raw materials for cordierite, aluminumtitanate, sialon, kaolin, talc, etc.

The ceramic material may be any of various crystalline substances (e.g.α-crystalline substances and β-crystalline substances) and amorphoussubstances and may be a mixture of two or more kinds of crystal systems.

The binder used in the third embodiment is the same as described in thefirst embodiment, but preferably contains, as a major component, ahydroxypropyl methyl cellulose having a water separation ratio of 40% ormore because a raw material powder of high electrical conductivity isused.

In the third embodiment of the present invention, then, in the samemanner as in the second embodiment, the above-mentioned mixed rawmaterial is mixed with water, followed by kneading, to obtain a plasticmixture; the plastic mixture is molded into a honeycomb shape to obtaina green honeycomb structure.

The preparation and molding of the plastic mixture are conducted in thesame manner as in the first embodiment.

Then, the thus-obtained green honeycomb structure is dried.

The drying of the green honeycomb structure can be conducted by a stepcontaining at least one kind selected from the group consisting ofhot-air drying, microwave drying, dielectric drying, reduced pressuredrying, vacuum drying and freeze-drying.

In particular, there is preferred a step containing hot-air dryingcapable of removing a liquid medium in the green honeycomb structure by35% or more and, for quicker drying, there is more preferred a stepfurther containing microwave drying and/or dielectric drying.

In the third embodiment of the present invention, lastly, the driedhoneycomb structure obtained by the above drying step is fired.

The method of firing, the firing conditions, etc. are the same as in thefirst embodiment.

The present invention is described more specifically below by way ofExamples. However, the present invention is in no way restricted bythese Examples.

EXAMPLE 1

A mixed raw material consisting of 300 kg of silicon carbide (SiC) as aceramic raw material, 25 kg of a binder composed of a hydroxypropylmethyl cellulose having a water separation ratio of 37% and 1.2 kg of asurfactant (potassium laurate) was kneaded with 85 kg of water for 50minutes using a kneader. The kneaded material was molded into acylindrical shape using a vacuum pug mill. The cylindrical shape waspassed through a ram extruder to obtain a honeycomb structure having adiameter of 150 mm, a slit width of 0.3 mm and a cell density of 47cells/cm². The green honeycomb structure obtained was cut into a lengthof 200 mm, followed by drying for 15 minutes using a microwave drier of1.6 kW.

Then, the resulting material was dried at about 120° C. for 2 hoursusing a hot-air drier to remove the water in the green honeycombstructure completely to obtain a dried honeycomb structure.

EXAMPLE 2 to 18 AND COMPARATIVE EXAMPLES 1 to 6

Dried honeycomb structures were obtained in the same manner as inExample 1 except that raw materials and binders shown in Tables 1 and 2were used and microwave drying was conducted for times shown in Table 2.

TABLE 1 Viscosity of 2% Methoxyl Hydroxypropoxyl Water separation Kindof aqueous solution group group ratio binder Binder composition at 20°C. (mPa · s) (wt. %) (wt. %) (%) A Hydroxypropyl methyl cellulose 2900028.1 5.7 37 B Hydroxypropyl methyl cellulose 4000 29.0 9.3 42 CHydroxypropyl methyl cellulose 28000 29.5 9.4 46 D Methyl cellulose 800028.4 — 25 E Hydroxyethyl cellulose 5000 — — No gel formation andmeasurement was impossible. F Carboxymethyl cellulose 700 — — No gelformation and measurement was impossible.

TABLE 2 Time of Ratio of Ratio of Kind of microwave microwave hot-airCracks on Raw material powder binder used drying (min) drying (%) drying(%) drying Ex. 1 SiC A 15 55 45 No Ex. 2 SiC B 14 50 50 No Ex. 3 SIC C16 58 42 No Ex. 4 SiC: 70 wt. % + Si3N4: 30 wt % B 12 45 55 No Ex. 5SiC: 50 wt % + glass: 50 wt % A 16 65 35 No Ex. 6 SiC: 70 wt % + AiN: 30wt % B 14 51 49 No Ex. 7 SiC: 70 wt % + TiC: 30 wt % C 13 48 52 No Ex. 8SiC: 70 wt % + ZrC: 30 wt % C 14 46 54 No Ex. 9 SiC: 70 wt % + B4C: 30wt % B 16 59 41 No Ex. 10 SiC: 80 wt % + Si: 20 wt % B 19 63 37 No Ex.11 SiC: 60 wt % + Si: 40 wt % C 19 55 45 No Ex. 12 SiC: 40 wt % + Si: 60wt % C 16 47 53 No Ex. 13 SiC: 80 wt % + Cu: 20 wt % C 18 56 44 No Ex.14 SiC: 80 wt % + Al: 20 wt % C 17 57 43 No Ex. 15 SiC: 80 wt % + Fe: 20wt % C 17 52 48 No Ex. 16 SiC: 80 wt % + Ni: 20 wt % C 17 51 49 No Ex.17 Raw materials for cordierite C 12 53 47 No Ex. 18 Alumina C 10 41 59No Comp. Ex. 1 SiC D 15 56 44 Yes Comp. Ex. 2 SiC: 80 wt % + Si: 20 wt %D 13 44 56 Yes Comp. Ex. 3 Raw materials for cordierite D 17 78 22 NoComp. Ex. 4 Raw materials for cordierite D 12 55 45 Yes Comp. Ex. 5 Rawmaterials for cordierite L 13 60 40 Yes Comp. Ex. 6 Raw materials forcordierite F 12 57 43 Yes Raw materials for cordierite: kaolin: 48 wt% + talc: 39 wt % + alumina: 13 wt%Evaluation Methods and Evaluation1. Evaluation Methods(1) Microwave Drying Ratio and Hot-Air Drying Ratio

In Examples 1 to 18 and Comparative Examples 1 to 6, each greenhoneycomb structure obtained from an extruder was cut into a length of200 mm, and the weight (W1) was measured at that point. Then, thestructure was dried using a microwave drier and the weight (W2) wasmeasured at the time when microwave drying was completed. Lastly, dryingwas conducted using a hot-air drier and the weight (W3) was measured atthe time when hot-air drying was completed. Using these weights (W1, W2and W3), microwave drying ratio and hot-air drying ratio were determinedfrom the following formulae (2) and (3):Microwave drying ratio=(W 1−W 2)/(W 1−W 3)×100   (2)and,Hot-air drying ratio=(W 2−W 3)/(W 1−W 3)×100   (3).(2) Generation of Cracks

After the completion of drying using a hot-air drier, generation ofcracks in dried honeycomb structure was observed visually.

2. Evaluation

In each of Examples 1 to 18 wherein hydroxypropyl methyl cellulose wasused as a binder, the hot-air drying ratio was 35% or more but themolded structure after drying generated no cracks. In particular, inExamples 10 to 16 wherein there was used a raw material powdercontaining a metal powder and having an electrical conductivity higherthan in Examples 1 to 9, there was used a binder B or C each consistingof a hydroxypropyl methyl cellulose having a water separation ratio of40% or more and therefore a crack-free dried material could be obtainedalthough the hot-air drying ratio was 35% or more.

In contrast, in Comparative Examples 1 to 6 wherein no hydroxypropylmethyl cellulose was used as a binder, cracks generated in allComparative Examples other than Comparative Example 3, wherein thehot-air drying ratio was 35% or more, although no cracks generated inComparative Example 3 wherein raw materials for cordierite were used asa ceramic raw material, the microwave drying ratio was 78% and thehot-air drying ratio was 22%.

Industrial Applicability

According to the process for producing a honeycomb structure accordingto the present invention, a crack-free high-quality honeycomb structurecan be produced rapidly at a low cost. Therefore, the present process isthought to have high industrial applicability.

1. A process for producing a honeycomb structure, which comprises addingwater to a mixed raw material of a raw material powder and a binder,kneading resulting mixture to obtain a plastic mixture, molding theplastic mixture into a honeycomb shape to obtain a green honeycombstructure, drying the structure by a step including hot-air drying, andfiring resulting dried honeycomb structure, to form a porous honeycombstructure, wherein the binder contains hydroxypropyl methyl cellulose asa major component.
 2. A process for producing a honeycomb structureaccording to claim 1, wherein the hydroxypropyl methyl cellulose has awater separation ratio represented by the following formula (1), of 40%or more:water separation ration=(Wb−Wa)/(Wb)×100   (1) (in the formula, Wa is agel weight after water separation and Wb is a gel weight before waterseparation).
 3. A process for producing a honeycomb structure accordingto claim 2, wherein the binder further contains at least one kindselected from the group consisting of methyl cellulose, hydroxyethylcellulose, carboxymethyl cellulose and a polyvinyl alcohol.
 4. A processfor producing a honeycomb structure according to claim 1, wherein thedrying of the green honeycomb structure includes a step of conductinghot-air drying to remove 35% or more of the water present in the greenhoneycomb structure.
 5. A process for producing a honeycomb structureaccording to claim 4, wherein the drying of the green honeycombstructure further includes a step of conducting microwave drying and/ordielectric drying to remove the water present in the green honeycombstructure.
 6. A process for producing a honeycomb structure according toclaim 1, wherein the mixed raw material is obtained by mixing 68 to 98%by weight of the raw material powder, 2 to 15% by weight of the binderand 0 to 30% by weight of other additives.
 7. A process for producing ahoneycomb structure according to claim 6, wherein the raw materialpowder contains 50% or more of SiC.
 8. A process for producing ahoneycomb structure according to claim 6, wherein the raw materialpowder contains SiC and a metal in a total amount of 90% or more.
 9. Aprocess for producing a honeycomb structure according to claim 6,wherein the metal contains at least one kind selected from the groupconsisting of copper, aluminum, iron, nickel and silicon.
 10. A processfor producing a honeycomb structure according to claim 6, wherein theother additives include at least one of a crystal growth aid, adispersing agent, and a pore former.
 11. A process for producing ahoneycomb structure according to claim 10, wherein the crystal growthaid includes at least one of magnesia, silica, yttria and iron oxide.12. A process for producing a honeycomb structure according to claim 10,wherein the dispersing agent includes at least one of ethylene glycol,dextrin, fatty acid soap and polyalcohol.
 13. A process for producing ahoneycomb structure according to claim 10, wherein the pore formerincludes at least one of graphite, wheat flour, starch, phenolic resinand polyethylene terephthalate.